1. Field of the Invention
This invention relates to a metallization method applied to preparation of semiconductor devices. More particularly, it relates to a method for forming a contact region having a barrier metal structure with low resistivity, high barrier properties and excellent filling with Al metallization material.
2. Description of the Related Art
Recently, the design rule for semiconductors is becoming finer, as exemplified by VLSIs and ULSIs of recent origin. Concomitantly, the junction is becoming shallower and the aspect ratio of the interconnection hole for establishing electrical connection between an upper metallization and a lower metallization is becoming higher. The upper metallization is usually formed by depositing an Al based material by sputtering. However, under the refined current design rule, malfunctions such as destruction or deterioration of the junction or increase of the contact resistance tend to occur more frequently than heretofore due to Al elution from the upper metallization towards a diffusion layer of the lower metallization or segregation of silicon (Si), usually added to the Al-based material, into contact holes. On the other hand, under the usual sputtering method, it is no longer possible to charge the Al based material with sufficient step coverage into the contact hole having an aspect ratio higher than unity, thus producing breakage of the upper metallization.
It has become customary to provide barrier metal between the upper metallization and the lower metallization as means for preventing an alloying reaction between the upper metallization and the lower metallization or Si segregation. The barrier metal is usually formed of refractory metal silicides or alloys thereof, besides transition metals and transition metal compounds, such as nitrides, carbides, oxynitrides or borides of the transition metals. The barrier metal is provided not only in a single layer, but in a stack of different types of layers.
As a typical example, a dual layer barrier metal structure composed of a stack of a Ti layer and a TiN layer, looking from the substrate side towards the layer of the Al-based material (Ti/TiN based stack) may be used.
The Ti layer exhibits high affinity to oxygen so that it is capable of reducing a native oxide film formed on the surface of an impurity diffusion layer and hence proves to be an excellent material from the viewpoint of stably achieving low resistance ohmic contact. The Ti layer is also excel lent in wettability relative to the Al based material, so that, when the interconnection hole is to be filled with the Al-based material, surface migration of Al atoms may be promoted to achieve uniform filling. However, if used alone, the Ti layer can not satisfactorily fulfil the function as the barrier metal. The reason is that, if a Ti layer is interposed alone between the silicon substrate and the Al-based material layer, both the reaction between Si and Ti and that between Ti and Al proceed simultaneously, so that it is impossible to prevent Al punch-through, that is an aluminum spike.
The TiN layer, on the other hand, is thermodynamically stable with respect to Si and exhibits higher barrier properties than the Ti layer. However, the TiN layer also undesirably exhibits high contact resistance relative to p-type Si. Besides, when the TiN layer is deposited by a vacuum thin film deposition technique, it has the crystal size of the order of 20 nm, while the crystal has a columnar structure, so that, after heat treatment, Al is dispersed into the grain boundary and hence the Al spike again can not be prevented satisfactorily. Besides, if the TiN layer is deposited directly on the silicon substrate, oxygen taken into the layer as impurities tends to be segregated in an interface between the TiN layer and the silicon substrate, so that it is difficult for the TiN layer used alone to provide a low resistance ohmic contact.
For this reason, in the Ti/TiN system, the Ti layer is deposited first on a Si substrate and the TiN layer is subsequently deposited to make the best use of the merits proper to the two layers.
More recently, a two-layer barrier metal composed of a Ti layer and a TiON layer (Ti/TiON system), produced by introducing oxygen during deposition of the TiN layer, has been proposed a view to improving the effects of preventing Al diffusion in the TiN grain boundary by oxygen segregation in the boundary.
On the other hand, a high temperature bias sputtering has recently been proposed as means for combatting insufficient step coverage by the upper metallization. This method resides, as introduced in monthly "Semiconductor World", 1989, December issue, pages 186 to 188 (published by Press Journal KK), heating a wafer to hundreds of degrees centigrade, by a heater block, and effecting sputtering under application of an RF bias by the heater block. With this method, step coverage may be improved by the Al fellow effects and 1on bombardment under the effect of the bias voltage to allow production of a layer of the Al-based material having a planar surface.
It may appear that, when forming a contact having a barrier metal structure, high barriering properties and superior filling by the Al-based upper metallization may be realized simultaneously by using a Ti/TiON based material as a barrier metal and effecting high temperature bias sputtering. However, in effect, this combination leads to the following disadvantages.
The first disadvantage is that the TiON layer exhibits a sheet resistance higher by more than one digit than that of the oxygen-free TiN layer.
The second disadvantage is that after-corrosion becomes more likely to be produced than when the TiN layer is employed. The reason is that BCl.sub.3, usually added to a dry etching gas for the Al-based material layer and the barrier metal for reducing a native oxide, is reacted with oxygen in the TiON layer to produce Cl.sub.2. Structural factors also add up to such chemical factor to produce the after-corrosion. That is, the TiON layer has rough surface morphology and is inferior to the TiN layer in wettability with respect to the Al-based material, so that the TiON layer tends to provide a site of residence of residual chlorine in an interface between the TiON layer and the layer of the Al-based material.
The third disadvantage is that the TiON layer has a rough surface morphology and is inferior in wettability and reactivity with respect to the Al-based material, so that it is difficult for the fine interconnection holes to be filled uniformly.
It is now assumed that, as shown in FIG. 1, an interlayer insulating film 3 having an interconnection hole 4 is stacked on a Si substrate 1, so that the contact hole reaches an impurity-diffusion region 2 previously formed in the Si substrate 1, and a Ti layer 5 and a TiON layer 6 are stacked as a barrier metal in this order for covering at least the interconnection hole 4. If it is attempted to deposit a layer of an Al-based material 7 on the wafer by high temperature bias sputtering, the interconnection hole 4 can not be filled uniformly, but voids 8 tend to be produced. The reason is that Al is in the intermediate state between a solid state and a liquid state during the high temperature bias sputtering and is highly sensitive to surface morphology of the underlying layer. That is, the TiON layer 6 has a columnar structure and has its crystals so oriented that the longitudinal direction thereof lies substantially orthogonally with respect to the film surface, so that the TiON layer exhibits rough surface morphology.
Thus the present inventors made attempts of stacking a Ti layer, which has proven reactivity and wettability with respect to the Al-based material, on the TiON layer 6, with a view to providing a Ti/TiON/Ti three-layer barrier metal structure. However, surface morphology could not be improved sufficiently by the newly deposited Ti layer, so that the interconnection hole 4 could not be filled with the Al-based material uniformly and with good reproducibility.